The invention described herein relates to nuclear reactor fuel rods and more particularly to an improved apparatus and process for welding an end plug in the end of a reactor fuel rod.
As is well known in the art, after nuclear fuel pellets are loaded into a fuel tube, end plugs then placed in opposite ends of the tube are appropriately welded to the tube at the seam formed by abutting surfaces at the end plug-fuel tube interface to thus form a fuel rod. One of the plugs has an axial opening therein to permit evacuating the fuel rod of gases prior to commencement of and during the welding operation. Upon completion of the girth weld at the seam, the axial opening in the plug is welded shut in a pressure chamber to maintain the internal area of the fuel rod under a high pressure.
According to current practice, the end plug is welded onto the fuel tube by inserting the plugged tube into a weld chamber and against a rotary stop therein, the distances being arranged such that the welding electrode tip is located at a fixed point relative to the tube-plug seam or joint. At present, this point is located on the plug and approximately 0.030 inch away from the tube-plug seam in order to comply with established, qualified weld parameters. It has been determined that in the event the fuel rod is not accurately positioned in the stop member in the weld chamber, a small displacement away from the end plug-fuel tube seam adversely affects the amount of weld penetration into the metal at the seam. Analysis of the welding operations carried out show that on occasion, the welding operator does not push the end plug firmly against the stop in the welding chamber. As a result, girth welds made in an area which falls outside that area where effective welds are obtained, often results in a defective weld because the amount of penetration is not sufficiently great to provide a tight seal at the end plug-fuel tube seam or joint.
According to conventional practices, a rotating chuck grasps and rotates the fuel rod during the time the girth weld is being made. It has been found that after the chuck grasps the fuel rod, there is a tendency for the chuck to pull the fuel rod away from the stop member against which the fuel rod end plug has been placed. This action results in an unwanted displacement, and in some instances, a displacement of as much as 0.006 inch away from the seam, has resulted in defective welds because proper weld penetration does not occur at the fuel tube-end plug interface.